Optical encoders are used in a wide variety of contexts to determine movement and/or a position of an object with respect to some reference. Optical encoding is often used in mechanical systems as an inexpensive and reliable way to measure and track motion among moving components. For instance, printers, scanners, photocopiers, fax machines, plotters, and other imaging systems often use optical encoding to track the movement of an image media, such as paper, as an image is printed on the media or an image is scanned from the media.
One common technique for optical encoding uses an optical sensor and an encoder pattern (or encoding media). The optical sensor focuses on a surface of the encoder pattern. As the sensor moves with respect to the encoder pattern (or encoding media), or the encoder pattern moves with respect to the sensor, the sensor reads a pattern of light either transmitted through, or reflected by, the encoder pattern to detect the motion.
A typical encoder pattern is an alternating series of features. As the encoder and sensor move relative to the one another, transitions from one feature to the next in the pattern are optically detected. For instance, an encoder pattern could be an alternating pattern of holes, or optically transmissive windows, in an opaque material. In which case, an optical sensor can detect transitions from darkness to light passing through the holes or windows.
FIG. 1 illustrates a basic optical encoder 100 comprising an optical unit 103 including a light emitter 101 and an optical sensor 102, and a light controlling member (encoder pattern) 105 disposed between the light emitter 101 and the optical sensor 102. The optical unit 103 and the encoder pattern 105 can move relative to each other in a linear fashion longitudinally of the encoder pattern 105. For example, if the optical member 103 is mounted on the printing head of the inkjet printer and the encoder pattern 105 is fixed to a case of the inkjet printer, the optical unit 103 moves along the length of the encoder pattern 105 when the printing head moves.
FIGS. 2A and 2B illustrate in further detail a 3-channel optical encoder comprising an encoder pattern and an optical sensor. FIG. 2A shows a code wheel (or code strip) encoder pattern 105, comprising an alternating A/B pattern 210 and a separate index pattern 220. The A/B pattern 210 comprises a pattern having alternating areas of differing optical transmissivity or reflectivity, depending on the design of the optical encoder. All of the first (“A”) portions 212 have a same length longitudinally along the encoder pattern 105. All of the second (“B”) portions 214 have a same length longitudinally along the encoder pattern 105. Further, the length of a first portion 212 longitudinally along the encoder pattern 105 and the length of a second portion 214 longitudinally along the encoder pattern 105 are equal to each other. In other words, a length L as a sum of the length of a first portion 212 longitudinally along the encoder pattern 105 and the length of a second portion 214 longitudinally along the encoder pattern 105 is constant in any adjacent pair of first and second portions 212, 214.
Meanwhile, FIG. 2B shows a optical sensor 102, comprising an A/B channel detector array 250, first and second (“A” and “B”) channel processing blocks 270 and 275, index channel detector 280, and index channel processing block 290. Although not shown in FIGS. 2A-B, the optical encoder also includes a light emitter such as light emitter 101 of FIG. 1. The light emitter may comprise one or more light emitting diodes for illuminating the encoder pattern 105.
The optical sensor 102 includes, as shown in FIG. 2B, a photodiode group 260 including four photodiodes 261-264. These four photodiodes 261-264 are arranged close to each other in the direction of the relative movement between the optical unit 103 and the encoder pattern 105. All of the photodiodes 261-264 have a same length in the direction of the arrangement, and a total of the four lengths is K. In other words, a length of the photodiode group 260 in the direction of the relative movement between the optical unit 103 and the encoder pattern 105 is K. In the above arrangement, K and L are each other, for example equal to each other within a manufacturing tolerance or error. It should be noted here that there may be a slight gap between each pair of adjacent photodiodes 261-264 due to technical reasons of manufacture, but these gaps are not illustrated in FIG. 2B. The photodiodes 261-264 have output terminals connected with input terminals of first and second channel processing blocks 270 and 275.
FIG. 3 more clearly illustrates a positional relationship between the encoder pattern 105 and the A/B channel detector array 250 including a plurality of photodiode groups 260.
If the encoder pattern 105 moves longitudinally with respect to the optical unit 103 (or vice versa), the first and second channel processing blocks 270 and 275 output the first and second channel signals “A” and “B” respectively, as shown in FIG. 4.
As can be seen in FIG. 4, the first and second channel signals indicate a relative movement between the optical unit 103 and the encoder pattern 105. However, they do not indicate a positional relationship of the optical unit 103 and the encoder pattern 105 with respect to each other. In many cases it is necessary to detect a relative positional relationship between the optical unit 103 and the encoder pattern 105 to determine an index or homing position of the optical sensor 102, or for end-point detection of the allowable movement range of the optical unit 103.
Accordingly, as shown in FIGS. 2A-B, the 3-channel optical encoder includes an index channel in addition to the first and second (“A” and “B”) channels. That is, the encoder pattern 105 includes the separate index pattern 220, and the optical sensor 102 includes the index pattern detector 280. The index pattern detector 280 outputs the index channel signal shown in FIG. 4.
However, there are some disadvantages to the 3 channel optical sensor as described above. For example, as optical encoders are increasingly integrated into smaller devices such as cameras, mobile telephones, etc., it becomes increasingly important to minimize the size of the optical encoder. However, the need for a separate index channel detector for the index channel increases the size of the integrated circuit for the optical unit. Similarly, the separate index pattern increases the size of the encoder pattern and the media (e.g., codestrip or codewheel) on which it is incorporated.
What is needed, therefore is an optical encoder that overcomes at least the shortcomings of known optical encoders described above.